The invention relates to a circuit for stabilizing a high voltage for a picture tube having a high-voltage transformer, which has a primary winding and, on the secondary side, a high-voltage winding, having a switch, which is connected in series with the primary winding, and having a control loop for high-voltage stabilization. Circuits of this type are used for example in television sets or computer monitors for the purpose of generating a stable high voltage.
EP 0 582 599 B1 discloses a circuit of this type which has a control loop between the high voltage on the output side and the emitter of the switching transistor. In this case, a control signal is tapped off from the high voltage by means of a resistive voltage divider and fed to a pulse width modulator. The latter uses the pulse-width-modulated signal to drive a transistor via which the emitter of the switching transistor is connected to a negative voltage, an inductance being connected between the emitter and the transistor. A capacitor with respect to which the switching transistor operates is connected between the emitter of the switching transistor and earth. The base of the switching transistor is driven, via a transformer, with a voltage which is line-synchronized and turns the switching transistor on during a line flyback.
DE-A 39 31 372, EP 0 592 151 B1, EP 0 483 432 B1 and EP-A 0 414 184 disclose further circuits for stabilizing a high voltage.
The object of the present invention is to specify a circuit for generating a stable high voltage for a picture tube which operates in a wide load range.
The circuit for stabilizing a high voltage for a picture tube comprises a high-voltage transformer, which has a primary winding and a high-voltage winding, a switch, which is connected in series with the primary winding, and a control loop for high-voltage stabilization. The control voltage of the control loop, with which the switch is driven, is derived from two signals in this case. The first signal is tapped off from a secondary auxiliary winding at the transformer and supplies a static control voltage component, and the second signal is derived directly from the high voltage and supplies a measure of the dynamic loading on the high-voltage generator.
The static control voltage component is, in particular, a measure of the value of the high voltage during the vertical field flyback, when the screen is black and the high-voltage transformer is therefore minimally loaded. In this case, the auxiliary winding supplies a voltage which is proportional to the secondary flyback pulse. The second signal is a fast control signal, since it is derived directly from the high voltage. This signal can advantageously be derived from the high voltage via a capacitive voltage divider, one capacitance being the aquadag capacitor that is present in any case in the picture tube, and the second capacitance being a low-voltage capacitor.
The first signal is detected by means of a time window during the line flyback and subsequently passed to a peak value rectifier, followed by an impedance converter. The output voltage of the latter is integrated and sampled synchronously with the vertical flyback pulse by a sample and hold circuit.
The output signal of the capacitive voltage divider is passed to a limiting network with an impedance converter and is reset by means of a switching logic unit after each frame or field by a reset switch which is synchronized with the vertical flyback pulse. The switching logic unit simultaneously controls the sample and hold circuit of the first signal. The output signals of the sample and hold circuit and of the reset circuit are passed to a difference-forming means, for example a differential amplifier, which generates the difference between the two signals and, as a result of this, the control signal for the switch.
The circuit has the advantage that, for the control, firstly a signal is used which is influenced very little by the load or loading (first signal via secondary auxiliary winding) and a second signal which follows the high-voltage loading in the microseconds range. At the same time, cost-effective standard components can be used for realizing the circuit. In particular, the controlling circuit does not require an expensive high-voltage divider block.
The stabilization circuit can be used in particular in high-voltage generators which provide high average picture powers, for example up to 100 watts for 36xe2x80x3 picture tubes, and are designed separately from the deflection circuit. For large picture tubes this enables a high picture brightness and a high picture contrast in conjunction with an improved picture geometry. It can be used in particular at relatively high deflection frequencies in the range of 30-50 kHz. Synchronization with the deflection frequency is optional in this case.